Image pickup apparatus

ABSTRACT

An image pickup system including an image pickup device (4) for taking an image from an object (1) containing a periodic pattern and outputting an image signal (5), a zoom lens (2) for forming an object image on the image pickup device, a moire amount detection circuit (10) for detecting an amount of moire from the image signal (5), and an imaging magnification control circuit including a differentiating circuit (18) provided for setting a magnification of the zoom lens (2) and finely adjusting the magnification of the zoom lens (2) to allow the amount of moire detected by the moire amount detection circuit (10) to become below the setting value, a positive/negative determination circuit (20), a system controller (22), and a ROM (24).

TECHNICAL FIELD

The present invention relates to an image pickup apparatus and, inparticular, to an image pickup apparatus applied to a defect checkingapparatus for checking a defect on a target object having a periodicpattern, such as a fabric, shadow mask and liquid crystal panel, and toa television camera (TV camera) for taking an image containing aperiodic pattern.

BACKGROUND ART

The fabric used for bags, shoes, conveyors, etc., is manufactured by aweaving machine and has a periodic pattern such as lattice--orinterdigital-stripes. Here it is to be noted that various defectsunavoidably occur to a certain extent in these fabrics depending uponthe homogeneity of those materials, working conditions, soiled spots inthe manufacturing process.

On the other hand, other products having a periodic pattern are, forexample, a shadow mask used in color CRTs and a liquid crystal panelused for liquid crystal display. The shadow mask is used as a colorseparation mechanism for the color CRT and comprised of an alloy platewith a periodic array of many fine openings for a plurality of colorcomponents provided to allow passage of electron beams of respectivecolor components. The liquid crystal panel is so constructed as to havea matrix array of many pixels. These products, such as a shadow mask andliquid crystal panel, sometimes suffer various defects in themanufacturing process.

The checking of defects in the products with a periodic pattern has beenmade by human eye.

In recent years, a defect checking apparatus has been developed whichautomatically checks defects by taking an image from an object with aperiodic pattern by means of a camera using a solid-state image pickupdevice such as a OCD image sensor and performing processing on an imagesignal obtained.

Taking the fabric as an example of an object, checking of a defect of arelatively large contrast, such as hole openings, burning marks, can beeffected relatively easily on the conventional defect checkingapparatus. However, it has been difficult to check microdefects specificto the fabric. That is, if the pixel pitch of the image pickup device ismade finer to accurately check such a microdefect, then a moire patternemerges as the size of the pixel gets nearer to a lattice of a periodicpattern of the object. If such a moire pattern appears, it becomesdifficult to check the object for defects and this provides a bar to thedetection of them.

Even on a TV camera such as a business TV camera for broadcasting inparticular, a high quality video camera for a general consumer and anHDTV (high definition television) camera, if an image is taken from anobject including a striped fabric having a fine periodic pattern, amoire interference emerges.

In order to avoid such a moire pattern, a conventional method is knownby which the high component of a spatial frequency involved is cut offby inserting an optical lowpass filter in an incident light path of theimage pickup device. However, this method involves a lowering in imageresolution. That is, with the optical lowpass filter inserted, very fineinformation of the high component of the spatial frequency is sacrificedso that such a camera cannot be applied to a defect checking apparatusof high precision adapted to check such a microdefects as set out above.For the TV camera, the lowering of the resolution would lead to thedegeneration of an image quality.

It is accordingly an object of the present invention to provide an imagepickup apparatus applicable to a defect checking apparatus and TV cameracapable of effectively eliminating moire interference withoutsacrificing an image resolution.

DISCLOSURE OF THE INVENTION

An image pickup apparatus according to the present invention comprisesan image pickup device for taking an image from an object containing aperiodic pattern and for outputting an image signal, image processingmeans for processing the image signal output from the image pickupdevice, and imaging magnification setting means for setting an imagingmagnification of the object relative to the image pickup device inaccordance with the pitch of the periodic pattern contained in theobject.

The image pickup apparatus according to the present invention furthercomprises, in addition to the above-mentioned arrangement, an imagingmagnification setting means for setting the imaging magnification of theobject relative to the image pickup device, amount of moire detectionmeans for detecting an amount of moire from the image signal, andimaging magnification adjusting means for adjusting the imagingmagnification set by the imaging magnification setting means so that theamount of moire detected by the amount of moire detecting means is madelower than the setting value.

Generally, the image pickup device is structured in a one- ortwo-dimensional array with a plurality of pixels arranged at apredetermined pitch. In the case where the image is taken by the imagepickup device from the object having the periodic pattern, a moirepattern emerges if the pixel pitch of the image pickup device is nearerto that of the periodic pattern imaged on the image pickup device.However, the moire pattern becomes zero by setting the pixel pitch in away to have an integral multiple of the pitch of the periodic patternimaged on the image pickup device. Further, when, in particular, thepixel pitch has a double relation to the pitch of the periodic patternimaged on the image pickup device, the highest resolution is obtained ina range in which no moire pattern emerges.

The pixel pitch of the image pickup device cannot be varied but thepitch of the periodic pattern imaged on the image pickup device can bevaried by varying the imaging magnification of the object relative tothe image pickup device.

With attention paid to this point, according to the present invention,the moire pattern is reduced by properly setting the imagingmagnification in a way to correspond to the pitch of the periodicpattern on the object. In this case, it is not necessary to set themoire pattern to completely zero and it is sufficient if being made at alevel lower than that detectable by the human eye. It is not necessarythat the pixel pitch be made precisely an integral multiple relation tothe pitch of the periodic pattern imaged on the image pickup device. Themoire pattern is adequately reduced even in that neighboring region. Forexample, in the case where the object, such as the fabric, contains avariation in pitch of the periodic pattern, it is not necessarilyrequired that, with the imaging magnification fixed, the moire patternbe reduced in all positions on the object. This is also true of theimage pickup apparatus, such as the TV camera for broadcasting, etc.,taking natural images. Even in an object, such as the shadow mask andliquid crystal panel, with the pitch of the periodic pattern exactlyuniform, there are sometimes the cases where irregular pitches occur atthe periodic pattern, with an image formed on the image pickup device,due to the deformation of a lens in an optical system by which an imageof the object is formed on the image pickup device.

According to the present invention, means is provided whereby, throughthe detection of the amount of moire from an image signal output fromthe image pickup device, the image magnification is adjusted so that thedetected amount of moire is reduced below a predetermined value, forexample, below a detectable limit. By this means it is possible to lowerthe moire pattern at all areas on the object.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an arrangement of an image pickupapparatus according to one embodiment of the present invention;

FIG. 2 is a block diagram showing a block diagram for realizing a majorsection of FIG. 1 by software processing;

FIG. 3 is a block diagram showing a practical form of an imageprocessing circuit in FIG. 1;

FIG. 4 is a view showing one example of a relation between pixels of animage pickup device and a periodic pattern used in the simulation of amoire pattern in the present invention;

FIG. 5 is a view showing another example of a relation between pixels ofan image pickup device and a periodic pattern used in the simulation ofa moire pattern in the present invention;

FIG. 6 shows a variation of a deviation value caused by a positionalvariation of the pixels when the periodic pattern of FIG. 4 and FIG. 5is taken at various pixel pitches;

FIG. 7 shows a variation of a deviation value when the pixel pitch isvaried with the pixel position fixed in FIG. 6;

FIG. 8 is a flow chart showing one example of the procedure ofcontrolling an imaging magnification in the embodiment;

FIG. 9 is a flow chart showing another example of the procedure ofcontrolling an imaging magnification in the present embodiment;

FIG. 10 is a view showing another example of an imaging magnificationvariable mechanism of the present invention;

FIG. 11 is a view showing still another example of an imagingmagnification variable mechanism of the present invention;

FIG. 12 is a view showing a schematic example of an aspect ratiocorrection mechanism at an image pickup optical system in the case wherean aspect ratio of a periodic pattern on the object is not 1:1 in thepresent invention; and

FIG. 13 is a view showing another example of aspect ratio correctionmechanism at an image pickup optical system in the case where the aspectratio of the periodic pattern on the object is not 1:1 in the presentinvention.

BEST MODE OF CARRYING OUT THE INVENTION

The present invention will be explained below in more detail withreference to the accompanying drawings.

The embodiment of the present invention will be explained below byreferring to the accompanying drawings. FIG. 1 is a schematic diagramshowing an image pickup apparatus according to one embodiment of thepresent invention.

In FIG. 1, an object 1 including a periodic pattern is comprised of, inthe case of a defect checking apparatus, a fabric, shadow mask, liquidcrystal panel or other objects to be checked and, in the case of a TVcamera for broadcasting for instance, a natural image. An image of theobject 1 is formed on an image pickup device 4 in a camera section 3through a zoom lens of an imaging amplification variable mechanism. Thezoom lens 2 is of a magnification variable lens system with a pluralityof optical lenses combined together as is well known in the art anddriven by a zoom driver 27 with the use of an electric motor as will beset out below to allow a variation of a magnification and, by doing so,a variation of an imaging magnification of the object 1 relative to theimage pickup device 4. Such a zoom lens having its magnificationcontrolled by the electric drive source is called a power zoom. Theimage pickup device 4 is comprised of a solid-state image pickup device,such as a one- or two-dimensional CCD (charged coupled device) imagesensor and delivers an electric signal (image signal) 5 corresponding toan image of an object thus formed.

The image signal 5 output from the image pickup device 4, beingamplified by an amplifier 6, is input to an A/D converter 7 andconverted to digital data (hereinafter referred to as image signal data)of, for example, a 8 bits-per-pixel, that is a 256-level, gradation. Theimage signal data is input to an image processing section 8 and, beingsubjected to predetermined processing, supplied to a display section 9,such as a CRT display and liquid crystal display, where it is displayedas an image.

The image signal data output from the A/D converter 7 is also input to amoire amount detection circuit 10.

The moire amount detection circuit 10 is adapted to detect an amount ofmoire components in the image signal 5 output from the image pickupdevice 4 and, in this example, comprises a normalization circuit 11 fornormalizing the image signal data from the A/D converter 7, aplural-stage shift register 12 for storing data corresponding to aplurality of continuous pixels of normalized image signal data, amaximum value/minimum value detection circuit 13 for detecting maximumand minimum values of the outputs of the stages of the shift register11, and a subtracter 16 for finding a difference between the maximumvalue 14 and the minimum value 15 output from the detection circuit 13.The output of the subtracter 16 thus obtained is delivered as an output17 of the moire amount detection circuit 10. The output 17 is used torepresent an amount of moire contained in the image signal 5 output fromthe image pickup device 1 as will be set out below. Hereinafter, theoutput 17 of the moire amount detection circuit 10 is referred to as amoire amount signal.

The moire amount signal 17 output from the moire amount detectioncircuit 10 is input to a differentiating circuit 18. The differentiatingcircuit 18 finds a differentiated value 19, that is, the slope of avariation of the moire amount signal 17 when the imaging magnificationof the zoom lens 2 is minutely varied, and is comprised of, for example,a digital difference device.

The output (differentiated values) 19 of the differentiating circuit 18is input to a positive/ negative determination circuit 20. Thepositive/negative determination circuit 20 determines a positive ornegative value (including zero) of the differentiated value 19 and aresult of determination, 21, is input, together with the moire amountsignal 17, to a system controller 22.

The system controller 22 is comprised of a circuit for controlling eachpart of the image pickup device on the basis of the moire amount signal17 output from the moire amount detection circuit 10, result ofdetermination by the positive/negative determination circuit 20 andsignals from a console 23 and ROM (read-only memory) 24. Here, out ofthe kinds of controls performed by the system controller 22, thefeaturing control of the present invention is the control made to thezoom driver 27.

In the case where the object 1 is comprised of an object to be checkedfor defects, an operator decides the kind of the object and sends thekind information, representing the kind of the object, to the systemcontroller 22. The kind information is comprised of the informationrepresenting the kinds of fabrics in the case of the object being thefabric, the information on the CRT's screen size (14 inches, 21 inches,29 inches, etc.), the information on the aspect ratio (4:3, 16:9, etc.)of the screen, and the information as to whether the CRT is a highdefinition use in the case of the object being the shadow mask of thecolor CRT, and the information on the panel size, number of pixels,etc., in the case of the object being the liquid crystal panel. Further,the operator can designate the field size of the image pickup device 4to the object 1 through the console 23. When the designation is so made,the information of the field is input to the system controller. Thefield size is defined by:

The pixel size×the number of pixels÷imaging magnification. The pitch ofthe periodic pattern contained in the object differs in values dependingupon the kinds of objects. The relation, that is, a relation between thekinds of objects to be checked and the basic pitch of the periodicpattern contained in the object, is initially determined and it isstored as a table in the ROM 24. Upon receipt of the kind informationfrom the console 23 the system controller 22 reads out the pitchinformation of the periodic pattern corresponding to the kindinformation from the ROM 24 and, on the basis of the pitch informationand information on the field size input from the console 23, sends animaging magnification control signal 25 to the zoom driver 27. The zoomdriver 27 sets the imaging magnification of the zoom lens 2 on the basisof the imaging magnification control signal 25 and, by doing so, setsthe imaging magnification of the object 1 to the image pickup device 4.For this reason, the imaging magnification corresponds to the pitchinformation of the periodic pattern and information on the field size.

After the imaging magnification has been set, the system controller 22slightly increases or decreases the imaging magnification control signal25, on the basis of the result of the positive/negative determination,21, from the positive/negative determination circuit 20 and the moireamount signal 17 from the moire amount detection circuit 10, so that theamount of moire is set below a setting value (detection limit). By doingso, the imaging magnification is finely adjusted. The practicalalgorithm for the fine adjustment of the imaging magnification will beexplained below.

Further, the monitor circuit 26 is provided in the present embodiment.The monitor circuit 26 informs a warming to the operator when theimaging magnification control signal 25 becomes such a value as todesignate an off-normal magnification (for example, a magnificationabove the upper limit or below a lower limit of the zoom) to the zoomlens 2. Partial processing can be performed on the moire amountdetection circuit 10, differentiating circuit 18, negative/positivedetermination circuit 20 and system controller 22 in FIG. 1 with the useof a software and, in the case, then that arrangement is as shown inFIG. 2. In FIG. 2, the image signal data output from the A/D converter 7in FIG. 1 is once stored in an image memory 31 and input to a CPU 33 viaa bus 32. In accordance with a program stored in ROM 34, the CPU 33performs the processing, by the software, corresponding to theprocessing done on the moire amount detection circuit 10,differentiating circuit 18, negative/positive determination circuit 20and system controller 22 in FIG. 1. A RAM 35 is comprised of a workingmemory for temporarily storing a result of part-calculation. Access canbe gained not via the CPU 33 but via a DMA controller not shown. It isto be noted that the ROM 34 can be shared also as the ROM 24 in FIG. 1.

For the case of the defect checking apparatus, the image processingcircuit 8 in FIG. 1 is so arranged as shown in FIG. 3. In FIG. 3, theimage signal data from the A/D converter 7 in FIG. 1 is branched intotwo paths, one leading to a first cumulative processing circuit 41 andthe other leading to a second cumulative processing circuit 43 via adelay circuit 42. The cumulative processing circuit 41 cumulates thedata corresponding to a plurality (N) of continued pixels (N=100 forexample) of input image signal data. The delay circuit 42 delays theinput image signal by a shifting operation by an amount correspondingto, for example, N pixels and the second cumulative processing circuit43 cumulates data of N pixels, like the first cumulative processingcircuit 41, in the image signal output form the delay circuit 42. Thatis, these cumulative processing circuits 41 and 43, each, cumulate the Npixels delayed behind the amount corresponding to the N pixels. Theresults of cumulation by the cumulative processings 41 and 43 are inputto a correlation calculation circuit 44 where an autocorrelation valueof both, such as a difference or ratio, is found.

Through the autocorrelation processing of the correlation calculationcircuit 44 in combination with the cumulative processing circuits 41, 43and delay circuit 42 a defect in the object 1 is obtained as enhancedimage data. On the image data output from the correlation calculationcircuit 44, threshold processing is performed, by a thresholdcalculation circuit 45, with the use of a proper threshold value and,through this determination, it is possible to extract data correspondingto a defect spot.

For the case of the TV camera for broadcasting, etc., the imageprocessing circuit 8 subjects the image signal from the A/D converter 7simply to conversion processing to obtain a signal of a formatdetermined by a desired TV standard system, such as an NTSC, PAL orHDTV.

The operation of the image pickup apparatus of the present embodimentwill be explained below. First, an explanation will be given of aprinciple of detection by the moire amount detection circuit 10.

A pattern 51 as shown in FIG. 4 or FIG. 5 is considered as a typicalexample of a periodic pattern in the object 1. The pattern 51 isprepared for simulating the generation of a moire pattern with verysmall solid squares (hereinafter referred to as black squares) formed ata 2C pitch in a white background, noting that one side of the flatsquare has a length C an 2C shows the pitch of the periodic pattern. Letit be supposed that such a pattern 51 is taken by the image pickupdevice 4. A square 52 shows a pixel of the image pickup device 4. Giventhat the size of the pixel 52 is represented by P×P (P hereinafterreferred to as the pixel pitch), the case of FIG. 4 corresponds to(2n-1)C≦P≦2nC and the case of FIG. 5 corresponds to 2nC≦P≦(2n+1)C,provided that n=any integer (1, 2, . . . ).

In this case, the luminance V of the image signal 5 output from theimage pickup device 4 can be expressed by the following equation (1).

    V=Zm {1-(4S.sub.B /P.sup.2)}                               (1)

Here, Zm denotes the maximum luminance, that is, the pixel value of theimage signal 5 corresponding to all white in the pixel 52 and "256"value of the output (8 bit data) of the A/D converter 7. S_(B)represents a whole area of the black square in the pixel 52 and P², anarea of the pixel 52.

When the luminance V is normalized by the maximum luminance Zm, a valueD as shown in the following equation (2) is obtained. The value D ishereinafter referred to as a deviation value.

    D=V/Zm =1-(4S.sub.B /P.sup.2)                              (2)

FIG. 6 shows a relation (variation), to the deviation value D, of anamount of X direction movement in the case where, with the pixel pitch Pvariously varied as C, 2C, 3C, 4C, 5C, 6C, . . . , the pixel 52 is movedin the X direction relative to the pattern 51 in conjunction with theperspective pixel pitch as indicated by arrows in FIG. 4 or 5. In FIG.6, X shows a position of a left upper end of the pixel 52 on the pattern51 in FIG. 4 or FIG. 5. The X-direction movement of the pixel 52corresponds to the main scanning of the image pickup device 4.

As evident from FIG. 6, the deviation value D greatly varies with theX-direction movement of the pixel 52 in the case where P=C, 3C, 5C(characteristics 61, 63, 64), that is, the pixel pitch P is an oddmultiple of the length of one side of the black square. This variationemerges as a moire pattern. On the other hand, the deviation value Dbecomes constant irrespective of the movement of the pixel 52 in thecase where P=2C, 4C, 6C (characteristics 62, 64, 66), that is, the pixelpitch P is an even multiple (an integral multiple of the pitch 2C of theperiodic pattern) of the length C of one side of the black square. Therearises no moire pattern. When, in particular, the pixel pitch P has atwo-times relation to the pitch of the periodic pattern imaged on theimage pickup device, the highest resolution arises in a range producingno moire.

FIG. 7 shows a variation of the deviation value D relative to avariation of the pixel pitch P in the case of X=C where the moirepattern is most liable to emerge.

From the result of the above simulation it is found that the moirepattern is eliminated by setting the pixel pitch P of the image pickupdevice 4 to be an integral multiple of the pitch 2C of the periodicpattern of the object 1 imaged on the image pickup device 4. However,even if there occurs the moire pattern no problem arises if it is to anextent not perceptible by human eye. It is, therefore, not necessarythat a relation of the pixel pitch P of the image pickup device 4 to thepitch 2C of the periodic pattern be made exactly an integral multiple inreal practice. To this end, in the embodiment of FIG. 1, an initiallyset imaging magnification may be so finely adjusted as to allow themoire pattern to be set to a limit not perceptible by human eye.

The relation of the pixel pitch P of the image pickup device 4 to thepitch of the periodic pattern of the object 1 imaged on the image pickupdevice 4 can be controlled by varying the imaging magnification of theobject 1 relative to the image pickup device 4, that is, themagnification of the zoom lens 2. In FIG. 7 the direction in which thepixel pitch P is greater corresponds to that in which the imagingmagnification is smaller in direction and the direction in which thepixel pitch P is smaller corresponds to that in which the imagingmagnifications is greater in direction.

The moire amount detection circuit 10 in FIG. 1, delivers a moire amountsignal 17 by finding the above-mentioned deviation value D by thenormalization circuit 11, storing the image signal data of an intervalcorresponding to a plurality of continuous pixels, by the shift register12, for example, the position of X=0 to 2C, finding a maximal value 14and minimal value 15, by the maximum value/minimum value detectioncircuit 13, with respect to the image signal and finding a difference ofthese by the subtracter 16. That is, as clear from FIG. 6, at a moirepattern emerging area the luminance varies periodically at the period ofbelow 2C and its variation amount, that is, the difference between themaximum value and the minimum value of the luminance, becomes greater ina way to correspond to the amount of moire, so that the amount of moirecan be detected through the above-mentioned processing (by finding thedifference of the maximum and minimum values during the 2C period) andit is possible to detect the amount of moire.

Although in the moire detection circuit 10 of FIG. 1, the amount ofmoire in the X direction only is detected in the case where the aspectratio of the pitch of the periodic pattern on the object 1, that is, theX- and Y-direction pitches, are equal, it may be possible to detect theamount of moire also in the Y direction in the case where the aspectratio of the pitch of the periodic pattern is other than 1:1, that is,the X- and Y-direction pitches are not equal to each other. In thiscase, the shift register 12 may be arranged in a plural array (N rows)so that it is possible to store image signal data corresponding to aplurality of continuous pixels in the Y-direction. Further, in the casewhere the TV camera takes natural images as an object, it is unclearwhat directionality the periodic pattern of the object possesses, thatis, in which direction the moire pattern emerges. It is desirable todetect the amount of moire in the x and y directions and, further, it iseffective to detect the amount of moire also in an oblique direction.

The practical procedure of controlling the imaging magnification in thepresent embodiment will be explained below.

FIG. 8 is a flow chart showing the procedure of controlling the imagingmagnification suitable to the case where the present invention isapplied to the defect checking apparatus, in particular, the case wherethe object to be checked has a random periodic pattern as that of thefabric.

When the operator sets the kind and field size of the object through theconsole 23 at step S11, at step S12 the system controller 22 sets theimaging magnification of the object relative to the image pickup device4 in accordance with the pitch information read out from the ROM 24 andin a way corresponding to the kind of the object and the field size setat step S11. The imaging magnification is so set as to be as great aspossible, so that microdefects can be detected with a higher resolutionunder the condition that the pixel pitch of the image pickup device 4becomes an integral multiple of the pitch (=2C) of a basic periodicpattern on the object imaged on the image pickup device 4.

By doing so, the imaging magnification control signal 25 is suppliedfrom the system controller 22 to the zoom driver 27 whereby themagnification of the zoom lens 2 is set.

Then at step S13, the system controller 22 checks whether or not themoire amount signal 17 output from the moire amount detection circuit 10is below the setting value, that is, whether or not the amount of moireis below a detection limit. When the amount of moire is below thedetection limit, the imaging magnification control is ended.

On the other hand, when at step S13 the amount of moire is determined tobe not below the detection limit, the system controller 22 minutelydecreases the magnification of the zoom lens 2 at step S14 by somewhatvarying the imaging magnification control signal 25. That is, the fineadjustment is made stepwise in a direction to decrease themagnification. The reason that the magnification is decreased at thefine adjustment is because a visual field is secured for defectchecking. That is, the decrease in the magnification leads to theincrease in the vision of field.

Then at step S15 the system controller 22 checks whether or not theamount of moire is below the detection limit as in the case of step S13.

When the amount of moire is below the detection limit, the imagingmagnification control is ended. When, on the other hand, the amount ofmoire is not below the detection limit at step S15 either, thepositive/negative determining circuit 20 determines whether adifferentiated value 19, that is, a variation caused by the fineadjustment of the imaging magnification of the moire amount signal 17detected by the differentiating circuit 18 at step S16, is positive ornegative (or zero).

Here, the case of the differentiated value 19 being positive means that,at FIG. 7, a deviation value D is located on the right side area Brelative to a position 0 (for example, P=2C, 4C, . . . , position) as anoptimal value and shifted in a direction away from the optimal point. Onthe other hand, the case of the differentiated value 19 being negativemeans that the deviation value D is located on the left hand side area Arelative to the optimal point position and shifted toward the optimalpoint. Further, the case of the differentiated value 19 being 0 meansthat the deviation value D is located at a boundary between the area Aand the area B, noting that this boundary is either at the optimal pointor worst point (P=C, P=3C, . . . , position).

When the differentiated value 19 is positive as a result ofdetermination at step S16, control is returned back to step S14. Untilat step S15 the amount of moire is determined as being in a detectionlimit range or at step S16 the differentiated value 19 is determined asbeing negative or 0, the imaging magnification is decreased in givenvery small amounts. That is, in the case where the differentiated value19 is positive, the differentiated value 19 becomes soon 0 or negativewith a decreasing imaging magnification as seen from FIG. 7 and,thereafter, the amount of moire is decreased to the detection limit.

On the other hand, when the differentiated value 19 is negative or 0 asa result of determination at step S16, checking is made to see whetheror not at step S17 the state of the amount of moire being above thedetection limit is sustained in a predetermined time, that is, at stepS14 the fine adjustment of the imaging magnification is effected apredetermined number of times. When the state above is sustained abovethe predetermined time, control is returned back to step S12 andre-setting of the imaging magnifications is carried out. Upon theresetting of the imaging magnification the imaging magnification is setto be smaller than that initially set at step S12 but in a rangesatisfying the condition of the integral multiple of the pitch 2C of theperiodic pattern on the object imaged on the image pickup device 4. If,for example, the imaging magnification initially set is a valuecorresponding to P=2C, re-setting is effected to a value correspondingto P=4C. Further, if the imaging magnification initially set is a valuecorresponding to P=4C, the imaging magnification is so re-set as tocorrespond to P=6C.

As shown in FIG. 7, the variation of the deviation value D resultingfrom the fine adjustment of the image magnification before and after theoptimal value (P=2C, 4C, 6C, . . . ) is greater at a greater imagingmagnification area (for example, P=2C) and is smaller at a smallerimaging magnification are (P=4C, 6C, etc.). According to the presentinvention, the fine adjustment is made at a smaller imagingmagnification and, therefore, it is easy to push the amount of moirebelow the detection limit.

As the imaging magnification is decreased, the resolution is lowered,thus lowering the detection capability per se relative to themicroracks. If, however, there arises any moire pattern, any relativegreat defect cannot be distinguished from the moire pattern, failing todetect it. With this taken into consideration, it is advisable to reducethe moire pattern through the lowering of the image magnification evenif the resolution is somewhat lowered.

And the above-mentioned processing is repeated hereinafter. As a resultof the above-mentioned processing, the control is ended when the moirepattern is below the detection limit.

Then the procedure of controlling the imaging magnification when thepresent invention is applied to the TV camera for broadcasting forexample will be explained below with reference to the flow chart asshown in FIG. 9.

First, at step S21, the operator (cameraman) of the TV camera manuallysets the imaging angle to set the imaging magnification, that is, themagnification of the zoom lens 2. At step S22, determination is made asto whether or not the amount of moire detected by the moire amountdetection circuit 10 is below the detection limit. Here, if the amountof moire is below the detection limit, the imaging magnification controlis ended without making any fine adjustment of the imagingmagnification.

When, on the other hand, the amount of moire is not below the detectionlimit, the imaging magnification is reduced by a predetermined verysmall amount at step S23 as in the case of the defect checking device asshown in FIG. 8 and then at step S24 determination is again made to seewhether or not the amount of moire is below the detection limit. When,as a result of determination, the amount of moire is below the detectionlevel, the imaging magnification control is ended and, when the amountof moire is not below the detection limit, a differentiated value 19 ofthe moire amount signal 17 is determined by the positive/negativedetermining circuit 20 at step S25 as to whether or not it is positiveor negative (or 0).

When, as a result determination at step S25, the differentiated value 19is negative or 0, control is returned back to step S23 and the imagingmagnification is decreased in predetermined very small amounts until atstep S24 the moire pattern is determined as being below the detectionlimit and at step S25 the differentiated value 19 is determined as beingpositive.

When at step S25 the differentiated value 19 is determined as beingpositive, the imaging magnification is increased, at step S26, by apredetermined very small amount and at step S27 determination is made tosee whether or not the amount of moire is below the detection limit.When at step S27 the amount of moire is below the detection limit, thecontrolling of the imaging magnification is ended and, when the amountof moire is not below the detection limit, control is returned back tostep S26 and, until the amount of moire becomes below the detectionlimit, the imaging magnification is increased by a predetermined verysmall amount.

It is not preferable, in the case of the TV camera, that the imagingangle intended by the cameraman is varied by the apparatus side so as toeliminate the moire pattern. In the case where, in the case of FIG. 9,unlike the case of FIG. 9, the amount of moire does not go below thedetection limit even in the case where the imaging magnification isadjusted in predetermined very small amounts toward a decreasingdirection, then reversely the imaging magnification is increased inpredetermined very small amounts toward a greater direction and theamount of moire is pushed below the detection limit. That is, by finelyadjusting the imaging magnification in the vicinity of the imagingmagnification corresponding to an imaging angle intended by thecameraman, the moire pattern is reduced. It is to be noted that, sincethe adjusting amount of the imaging magnification for moire reductionhas only to be very small, the angle never appears varied through thisadjustment.

In the case where, in the TV camera for taking natural images, theamount of moire is detected, as set out above, in a plurality ofdirections, such as the x-, y- and oblique-directions, it is ideal toeffect imaging magnification control as set out above so as to enablethe amount of moire in all the directions to go below the detectionlimit. It is also effective that imaging magnification control becarried out, if that is not possible, so as to enable the amount ofmoire in an visually outstanding direction (for example, X-direction) tobe lowered on a preferential way.

According to the present invention, it is possible to prevent occurrenceof the moire pattern by setting the imaging magnification of the objectimaged on the image pickup device in accordance with the pitch of theperiodic pattern on the object so as to enable the pixel pitch to bemade an integral multiple of the pitch of the periodic pattern imaged onthe image pickup device.

Other embodiments of the present invention will be explained below.

Although, in the embodiment shown in FIG. 1, the zoom lens 2 has beenexplained as being used as the imaging magnification variable mechanism,the imaging magnification variable mechanism is not restricted theretoand, for example, it may be so configured as shown in FIG. 10 or 11.

In FIG. 10, a camera moving mechanism 72 is provided as to enable acamera section 3 to be moved in an optical axis direction (in adirection perpendicular to the imaging surface of an image pickup device4) and imaging magnification is controlled by controlling the cameramoving mechanism 72 from a system controller 22 side in FIG. 1 by animaging magnification control signal 74. In this connection it is to benoted that the camera section 3, being moved toward the opticaldirection, goes out of focus relative to the image pickup device 4.Therefore, it is so configured that, by sending from the systemcontroller 22 to an image taking lens 71 a focal control signal 73varying in coincidence with the imaging magnification control signal 74,the distance between the image taking lens 71 and the image pickupdevice 4 is varied in interlock with the movement of the camera movingmechanism 72. In this case, as the image taking lens 71 use is made of alens capable of adjusting the focal point under electronic controlusing, for example, a motor.

In the arrangement as shown in FIG. 11A, as an imaging optical systemfor forming an object imaging on an image pickup device 4 in the camerasection 3, a lens exchange system 81 is used with a plurality of imagetaking lenses 82 of different focal points and an imaging magnificationis made variable by selectively inserting image taking lenses 82, onehaving a structure as shown in FIG. 11B and one as shown in FIG. 11C,into an incident optical path of the image pickup device 4. In theexample of FIG. 11B, a revolver type imaging magnification variablemechanism has a plurality of image taking lenses 82 arranged along acircumferential direction on a rotation disc 83 and is adapted to selectone image taking lens by rotating the disc 83 in a direction asindicated as an arrow. Further, in the example of FIG. 11C, a lineartype imaging magnification variable mechanism has a plurality of imagetaking lenses 82 arranged on a support plate 84 movable in a parallelway and is adapted to select one image taking lens by moving the supportplate 84 in a direction as indicated by an arrow.

Further, as the imaging magnification variable mechanism it is possibleto use the zoom lens 2 as shown in FIG. 1, camera moving mechanism 72 asshown in FIG. 10 and lens exchange system 81 as shown in FIG. 11 eitherselectively as the need arises or in any two or more componentcombination. It has been considered that, for example, the zoom lens isconvenient to use as the imaging magnification variable mechanism of thepresent invention because it can vary the magnification withoutbasically varying the focal point, but that the zoom lens is complex inlens structure, low in brightness and generally great in aberration.According to the arrangement as shown in FIG. 10 it is possible toeliminate the disadvantages involved in the case where the zoom lens isused. On the other hand, the camera moving mechanism 72, increasing itsvariable range, involves a bulkier mechanism. Using the zoom lens 2 andcamera moving mechanism 72 in combination eliminates this disadvantageand, since as the zoom lens 2 use can be made of one having a narrowmagnification variable range, it is possible to alleviate thedisadvantage possessed by the zoom lens.

In the image magnification variable mechanism for selecting the imaginglens as shown in FIGS. 11B and 11C, it is not possible to effect fineadjustment of the imaging magnification but, for the object with aperiodic pattern of a stable pitch, such as the shadow mask and liquidcrystal panel, it is considered unnecessary to make fine adjustment ofthe imaging magnification, so that such a stepwise imaging magnificationvariable mechanism may be used.

An explanation will be given below of the case where the periodicpattern pitch of the object 1 has an aspect ratio of not 1:1. Theperiodic pattern contained in the object, such as the fabric and liquidcrystal panel, is not always equal in the aspect ratio's pitch and oftendiffers is the aspect ratio's pitch. The above-mentioned imagingmagnification variable mechanism, of course, including the zoom lens,makes the imaging magnification variable at the 1:1 aspect ratio and, ifthe aspect ratio of the pitch of the periodic pattern in the object isnot 1:1, correction is required so that the aspect ratio of the pitch ofthe periodic pattern on the image signal output from the image pickupdevice 4 may be made at 1:1, that is, the aspect ratio per pixel of animage formed on the image pickup device 4 may be made at 1:1. FIG. 12and FIG. 13 each show an example of such aspect ratio correctingmechanism.

FIG. 12 shows an example of the image pickup device being aone-dimensional array sensor. An image of one line of an object 1 isformed by an image taking lens 91 on an image pickup device 4. Theobject 1 is made of a lengthy sheet as shown in FIG. 12 and moved by amoving mechanism 92 in a direction (sub scanning direction) as indicatedby an arrow y, the mechanism 92 being comprised of a roller set, etc.The moving mechanism 92 is so driven as to allow the object 1 to bemoved with a speed (sub scanning speed) corresponding to a sub scanningsynchronization signal 94 from the system controller 22 in FIG. 1. Theimage pickup device 4 is scanned by the scanning drive circuit 93 in adirection (main scanning direction) of a sensor as indicated by an arrowx and an image signal corresponding to one line is output in a serialfashion in accordance with the main scanning. The period of the mainscanning is controlled by a main scanning synchronization signal 95.

Given here that the aspect ratio of the pitch of the periodic pattern onthe object 1 is N:1 (N:a positive integer exceeding unity), theabove-mentioned sub scanning speed is made an N times its aspect ratioor the main scanning period is made at a 1/N ratio. By doing so, if N=2for example, the main scanning over one period of the image pickupdevice 4 is ended while the object 1 is moved in the sub scanningdirection by a distance corresponding to two pixels. By doing so, theaspect ratio of the pitch of the periodic pattern on the image signaloutput from the image pickup device 4, that is, the aspect ratio perpixel of an image formed on the image pickup device 4 becomes 1:1.

FIG. 13 shows an example of an image pickup device 4 being atwo-dimensional sensor. An image of an object 1 is formed on the imagepickup device 4 by a convex lens 101 and cylindrical lens 102 and theaspect ratio per pixel is corrected by the cylindrical lens 102.

That is, in the case where the aspect ratio of the pitch of the periodicpattern on the object 1 is 1:N (N:a positive number exceeding unity), ifthe axis of the cylindrical lens 102 is aligned with a y direction, animage formed on the image pickup device 4 is compressed in an xdirection. By properly selecting the curvature of the cylindrical lens102 the aspect ratio per pixel of an image formed on the image pickupdevice 4 is corrected to 1:1. In the case where, on the other hand, theaspect ratio of the pitch of the periodic pattern on the object 1 is N:1(N:a positive number exceeding unity), if the axis of the cylindricallens 102 is rotated in a direction to be aligned with the x direction,an image formed on the image pickup device 4 is compressed in the ydirection and, by properly selecting the curvature of the cylindricallens 102, the aspect ratio per pixel of an image formed on the imagepickup device 4 is corrected to 1:1.

By taking such an aspect ratio correcting mechanism in combination withthe image pickup device in FIG. 1, it is possible to detect theabove-mentioned moire amount even in the case where the aspect ratio ofthe pitch of the periodic pattern on the object 1 is not 1:1.

The present invention is not restricted to the above-mentionedembodiment and various changes and modifications can be made. Forexample, the present image pickup apparatus is not restricted as anapplication apparatus to the defect checking apparatus, etc., and can beapplied to anything if it is directed to taking an image of an objectincluding a periodic pattern. Although the aspect ratio of the pitch ofthe periodic pattern has been explained as being 1:1, it is notnecessarily required to be 1:1.

INDUSTRIAL APPLICABILITY

As already set out above, an image pickup apparatus of the presentinvention can take an image from an object having a periodic patternwith high precision while eliminating a moire pattern and be properlyapplied to a defect checking apparatus and a TV camera for broadcasting,etc.

We claim:
 1. An image pickup apparatus comprising:image pickup means fortaking an image from an object containing a periodic pattern and foroutputting an image signal; image processing means for processing theimage signal output from said image pickup means; and magnificationsetting means for setting an imaging magnification of the objectrelative to the image device in accordance with a pitch of a periodicpattern contained in the object, wherein said magnification settingmeans comprises means for setting the image magnification to allow apixel pitch of said image pickup means to be made an integral multipleof the pitch of the periodic pattern imaged on said image pickup means.2. The image pickup apparatus according to claim 1,wherein saidmagnification setting means comprises means for setting the imagemagnification to allow the pixel pitch of the image pickup means to bemade double the pitch of the periodic pattern imaged on said imagepickup means.
 3. The image pickup apparatus according to claim 1,wherein said magnification setting means comprises a zoom lens arrangedin an incident optical path of said image pickup means and means foradjusting the magnification of the zoom lens.
 4. The image pickupapparatus according to claim 1, wherein said magnification setting meanscomprises means for adjusting a distance between the object and saidimage pickup means.
 5. The image pickup apparatus according to claim 1,wherein said magnification setting means comprises plural lenses havingdifferent focal lengths and means for selectively inserting one of saidplural lenses into an incident optical path of said image pickup means.6. The image pickup apparatus according to claim 1, wherein saidmagnification setting means comprises memory means for storing arelation of the kind of objects to the pitch of the periodic patterncontained in the object and means for setting the imaging magnificationon the basis of the pitch read out from the memory means in accordancewith the kind of the object.
 7. The image pickup apparatus according toclaim 1, wherein said image processing means comprises means fordetecting a defect spot in the object.
 8. The image pickup apparatusaccording to claim 1, further comprising aspect ratio correcting meansfor varying an aspect ratio per pixel of the image formed on said imagepickup means by controlling at least one of a scanning speed of saidimage pickup means and a moving speed of the object relative to saidimage pickup means in accordance with the aspect ratio of the pitch ofthe periodic pattern contained in the object.
 9. The image pickupapparatus according to claim 1, further comprising aspect ratiocorrecting means for optically varying an aspect ratio per pixel of theimage formed on said image pickup means in accordance with the aspectratio of the pitch of the periodic pattern contained in the object. 10.An image pickup apparatus comprising:image pickup means for taking animage from an object containing a periodic pattern and for outputting animage signal; image processing means for processing the image signaloutput from said image pickup means; means for detecting an amount ofmoire on the basis of the image signal output from said image pickupmeans; and magnification setting means for setting an imagingmagnification of the object relative to said image pickup means so thatthe amount of moire detected by the detecting means becomes below asetting amount, wherein said magnification setting means comprises meansfor setting the image magnification to allow a pixel pitch of said imagepickup means to be made an integral multiple of a pitch of the periodicpattern imaged on said image pickup means.
 11. The image pickupapparatus according to claim 10,wherein said magnification setting meanscomprises means for setting the image magnification to allow the pixelpitch of said image pickup means to be made double the pitch of theperiodic pattern imaged on said image pickup means.
 12. The image pickupapparatus according to claim 10, wherein said magnification settingmeans comprises a zoom lens arranged in an incident optical path of saidimage pickup means and means for adjusting the magnification of the zoomlens.
 13. The image pickup apparatus according to claim 10, wherein saidmagnification setting means comprises means for adjusting a distancebetween the object and said image pickup means.
 14. The image pickupapparatus according to claim 10, wherein said magnification settingmeans comprises a plurality of lenses of different focal distances andmeans for selectively inserting any one of the plurality of image takinglenses in an incident optical path of said image pickup means.
 15. Theimage pickup apparatus according to claim 10, wherein said magnificationsetting means comprises memory means for storing a relation of kinds ofobjects to the pitch of the periodic pattern contained in the object andmeans for setting the imaging magnification on the basis of the pitchread out from the memory means in accordance with the kind of theobject.
 16. The image pickup apparatus according to claim 10, whereinsaid image processing means comprises means for detecting a defect spotin the object.
 17. The image pickup apparatus according to claim 10,further comprising aspect ratio compensating means for varying an aspectratio per pixel of the image formed on said image pickup means bycontrolling at least one of a scanning speed of said image pickup meansand moving speed of the object relative to said image pickup means inaccordance with the aspect ratio of the pitch of the periodic patterncontained in the object.
 18. The image pickup apparatus according toclaim 10, further comprising aspect ratio correcting means for opticallyvarying an aspect ratio per pixel of the image formed on said imagepickup means in accordance with the aspect ratio of the pitch of theperiodic pattern contained in the object.
 19. The image pickup apparatusaccording to claim 10, wherein said magnification setting meanscomprises means for re-setting an imaging magnification when the amountof moire detected by the detecting means is not below the settingamount.
 20. The image pickup apparatus according to claim 10, whereinsaid detecting means comprises means for detecting the amount of moireby finding a difference between a maximal value and minimal value of apredetermined range of the image signal.